Istanbul, Turkey — The disconnect between Tehran and world powers over Iran's nuclear program is so serious that talks scheduled for Moscow next week may be delayed, according to officials from both sides.

If the scheduled talks – the third round in just over two months – do take place June 18-19, says an Iranian diplomat close to the negotiations, they will be "really tough."

"Even if the Moscow talks start [on] time, I believe that it will be the scene of complaints for Iran and [world powers] and may finally agree to have a next round as a result," says the Iranian diplomat, contacted in Tehran. "But the question is: How long could it be continued like this?"

Officials from both sides accuse each other of not being "serious" about engagement, of stalling for time, and being unwilling to strike a deal that would calm Western and Israelis fear about Iran ever trying to build a nuclear weapon; lift crippling sanctions on Iran; and avoid possible military strikes by Israel or the US.

That call will come amid increasingly acrimonious exchanges that illustrate the stark challenges that remain to a negotiated solution between Iran and the P5+1 group (comprised of the US, Russia, China, England, France and Germany) represented by Ms. Ashton.

Iranian diplomat: 'Nothing but some vague replies'

The most recent letter late yesterday, from Mr. Jalili's deputy to Ashton's deputy, Helga Schmid, expressed "surprise" that Ms. Schmid's earlier letter "tabled issues that were a long way apart" from previous agreements and did not address Iran's counteroffer put forward in Baghdad in late May.

Jalili's deputy, Ali Bagheri, also asked why the P5+1 refused to meet at the experts level prior to the Moscow talks.

"If the agreements of each round of talks cannot be followed up at the level of deputies and experts, what guarantee will there be for the success of future talks?" Bagheri wrote in the letter that was leaked to Iranian media.

EU diplomat: 'Increasing negativity'

Diplomats from both sides claim that they have gone out of the way to accommodate the other in the interest of successful talks. But they also claim that they have "got nothing but some vague replies" (says the Iranian diplomat) or come up against "obfuscation" and "increasing negativity" (says a European diplomat in Brussels familiar with the talks).

In the past week there have been two direct calls between Bagheri and Schmid, notes the European diplomat. Since late May, five letters have been exchanged and "we have offered for at least a week a direct call between Ashton and Jalili, which they dragged their feet on."

Says the European diplomat: "Every effort has been aimed at ensuring that we reach out ... that we offer calls at a high level, that we communicate in every which way we can, [so] it is absurd to somehow shift the blame for an outcome in Moscow on a supposed unwillingness on our part [to engage]."

Iranian officials, in their turn, argue that it is they who are reaching out.

The Iranians "want ... to show that they are the ones who are interested in cooperating, and I believe highlighting the telephone conversations and sending letter after letter is just proof that they want to show the world that they are keen on talking and the P5+1 is the one refraining," says the Iranian diplomat.

"When Bagheri wrote in his letter that if they are not getting ready for the Moscow talks they should expect failure, [this] was paving the way ... to say to the world that we did send them the message and they ... ignored it," says the Iranian diplomat.

The top US negotiator in the nuclear talks, Undersecretary of State for Political Affairs Wendy Sherman, was scheduled to join other P5+1 officials for strategy talks in Strasbourg, France, today and tomorrow.

Besides the squabbling over preparations for Moscow is a more fundamental issue for Iran: the substance of the P5+1 opening bid put forward in Baghdad in late May, which Tehran says amounted to "failure."

In that proposal, Iran was asked to give up uranium enrichment to 20 percent purity – a level it has been converting into fuel plates for a research reactor, but which is technically not far from weapons-grade of 90 percent.

Iran was also asked to close the deeply buried Fordow facility, where the 20 percent enrichment work is currently under United Nations nuclear watchdog safeguards, and to agree to more intrusive inspections.

'Iran has lowered the tone and increased the volume'

But another demand was that Iran suspend all enrichment, as required by UN resolutions until it clears up questions of any weapons-related work.

While Iran has publicly stated a readiness to cap enrichment at 5 percent, it has also declared for years that it will not give up its "inalienable right" – as specified in the Nuclear Non-Proliferation Treaty (NPT) – to enrich uranium.

Iran's President Mahmoud Ahmadinejad said in a recent speech: "If some want us to forgo this right, they should first give their reasons, and second [disclose] what they will give the Iranian nation in return."

While those issues are expected to be at the heart of any deal, Iran has yet to specify in the talks what it will and will not accept, says the European diplomat.

"What [the Iranians] haven't done is [say], 'You know we can't do that, but what we would like to do is this. Can we as an initial step agree that?' says the European diplomat.

"That's not what we've been seeing, and that's why we're saying: 'Let's meet and have a conversation about that,'" he adds. "That's what we call an engagement over substance; that is exactly what the Iranians have not done. [Instead] they have postured, they've argued, they've lowered the tone and increased the volume."

The State Department said Iran had an “opportunity” in Moscow if it came “prepared to take concrete steps in response to the proposals presented in Baghdad.”

'[The P5+1] want Iran to give diamonds for peanuts'

Further complicating the picture for the Iranians is that the P5+1 proposal, while asking Iran to give up what it considers its most important cards, does not simultaneously provide what it most desires: lifting of crippling sanctions. A European oil embargo is due to begin on July 1.

Hossein Mousavian, a former member of Iran's nuclear negotiating team from 2003-05 who is now at Princeton University, told the Tehran Bureau website after the offer was made public last month: "They want Iran to give diamonds in return for peanuts."

That view is widely shared among decisionmakers in Tehran.

"On several occasions they have said, at least the Americans did, that Iran could have below 5 percent enrichment, but now they are asking for something else that is not acceptable to Iran," says the Iranian diplomat in Tehran familiar with the talks. "The next point is that ... they are not ready to ease the unilateral sanctions that are Iran's concern."

If Iran were to accept the P5+1 proposal as is, says the Iranian diplomat, "it would be a great win for them."

European and US officials have stated that their opening bid should be seen only as that, not a deal-breaker. In Iran, there is "confusion about what is an initial step and what is a fundamental step," says the European diplomat.

"We have seen through these exchanges a signpost that tells us how difficult this is going to be," he adds. "If anybody came out of Istanbul [the first round] feeling optimistic, they were given a boost of realism in Baghdad and another bucket of it afterwards."